Robert J. Borgman

Behavioral effects of apomorphine and diisobutyrylapomorphine in the mouse

Stereotyped climbing and clinging responses of the mouse to apomorphine or its ester prodrug, O,O′-diisobutyrylapomorphine were evaluated. Acute doses of the ester up to 0.3 mmoles/kg were tolerated without apparent ill effects. Both aporphines produced dose-dependent behavioral responses that were blocked by neuroleptics. The duration of action of the ester was much greater than that of apomorphine. When maximal initial behavior during the first hour was evaluated, low equimolar doses of apomorphine and the ester were similar in potency; in contrast, the total behavioral response to larger doses of the ester was greater than to apomorphine, evidently reflecting the greater duration of action of the ester. Behavioral responses to both agents during the first hour decreased at doses above 0.1 mmoles/kg. Oxidized or O-methylated apomorphine did not antagonize the behavioral effects of apomorphine. Systemic injection of apomorphine or diisobutyrylapomorphine led to detectable levels of free apomorphine as estimated by a sensitive and selective fluorimetric assay. The time-course and magnitude of the behavioral effects of both agents corresponded closely with brain levels of apomorphine. Apomorphine and dopamine (but not diisobutyrylapomorphine) stimulated adenylate cyclase activity in mouse striatal homogenates—an effect antagonized by neuroleptic drugs but not propranolol. Apomorphine exerted a biphasic effect on the cyclase in vitro and increased cyclic AMP levels in the striatum in vivo. The prolonged activity of apomorphine esters as depot prodrug agonists of putative dopaminergic mechanisms in the brain may provide clinically desirable characteristics.

Hydrolysis of diester prodrugs of apomorphine.

Abstract O,O′-diesters of apomorphine exert behavioral effects identical to those of apomorphine, but prolonged in proportion to the bulk of the esters. This prolonged activity may reflect depot properties of the esters, and/or decreasing rates of hydrolysis to the presumably active metabolite, apomorphine. To test the latter hypothesis, mouse brain apomorphine was assayed by a fluorimetric method that can distinguish apomorphine from its monophenolic analogues and was confirmed by thin-layer chromatography. Apomorphine fluorescence was recovered after systemic injection of di-isobutyrylapomorphine as well as after apomorphine, but its half-life in brain was greatly prolonged after administration of the ester. Ester hydrolysis also occurred in vitro with minces or homogenates of rat tissue, yielding a product with the fluorescent and Chromatographic properties of free apomorphine which also stimulated the activity of a dopamine-sensitive adenylate cyclase in rat striatal homogenates, sensitive to apomorphine but not its esters. Most of the activity resided in the high-speed supernatant fraction of liver; high esterase activity was found in rat plasma and was present in most tissues. This reaction was dependent on temperature, tissue and substrate concentration, followed saturable kinetics, and was inhibited by triorthocresylphosphate, an esterase inhibitor. This apparent enzymatic reaction proceeded at rates varying inversely with the size of the ester substituent. These results support the suggestion that the rate of hydrolysis may be related to desirable prolongation of action of ester prodrugs of apomorphine.

Prolonged decrease of serum prolactin in rat by an ester prodrug of apomorphine.

Abstract (1) An O , O ′-diester of apomorphine mimicked the prolactin-lowering action of apomorphine in the estrogen-primed male rat. (2) Its effects were somewhat slower in onset, but much longer-lasting than those of apomorphine. (3) These effects closely parallel previously observed long-lasting behavioral effects in rodents following the administration of apomorphine esters. (4) We believe these effects are mediated by central dopaminergic actions of apomorphine slowly liberated from these ester “prodrugs” by esterase hydrolysis.